A lot of stupid people in this comment section don't seem to understand the story. The entire plot is about how Murakami helps Moritani become a more outgoing person, and Moritani helps her remove the "mask" that she wears around others. Moritani is the only first person to see what she was like without that mask, and throughout the story she slowly figures out more and more of who Murakami is. She gets fed up with Murakami not being true to herself, and forces her to confront the problem. Similarly, Murakami forces Moritani to be more outgoing by forbidding her from escaping by time stopping unless it's for her.
It's a short, and tightly focused story. None of the characters outside the main two need any backstory or fleshing out, because it isn't important to the arcs of the main characters, who are the focus of the story. The time powers themselves aren't even that important. They only exist to bring the characters together and resolve their respective arcs. Them fading was a really clever way to tell the reader that the story is over, and nothing more needs to be said.